A radical enviornmentalist who took three hostages at the Discovery Channel headquarters while wearing what police may be explosives was shot and killed by officers, police said.

The gunman, identified as James Lee, was killed by police following four hours of negotiations but the hostages are all safe, said Montgomery County Police Chief J. Thomas Manger.

Manger said the suspect had "metalic canisters" strapped to his chest and back. When Lee was struck by police bullets, one of the canisters "popped." Police have not confirmed if the canisters were a bomb, but Manger said the "device may have gone off" when he was shot.

Manger said police will search the building looking for other potential explosives Lee may have left inside.

"All the hostages are safe," Manger said, ending a four hour standoff in which some 1,900 employees were evacuated from the building as well as the company's on-site daycare center.

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Hostage Crisis at Discovery Channel

Manger said police spoke with Lee "for several hours" during which time he expressed "a wide range of emotions during those communications."

Law enforcement identified the suspect as James Lee, 43, from Washington, D.C., who has for at least two years called for protests against the company and who was arrested and found guilty of disorderly conduct for a protest outside of Discovery's headquarters in 2008.

In a rambling manifesto on Lee's website, believed to have been written by Lee, the writer rails against "disgusting human babies," "parasitic infants," and says people should "disassemble civilization." The manifesto also calls on Discovery to "broadcast to the world their commitment to save the planet."

The drama began when workers in the building got a bulletin from the building's security.

"We have reason to believe there is an armed gunman at One Discovery Place. All employees should seek protection in a locked office on their respective floors immediately," read an email sent to employees and read over the public address system.

"There's a guy with a gun in the lobby. Police are swarming in -- assault rifles and all," one producer told ABCNews.com via instant message.

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Hostage Drama at Discovery Headquarters

Employees at the building were told to take cover in locked offices and police have sent SWAT teams to the area to seal off nearby roads.The FBI and agents from ATF, including explosives experts, also rushed to the building.

One witness, project manager Ari Fisher said he returned to the building at 1 p.m. following lunch and peered into the lobby before being told to flee from the building.

Fisher says he saw the gunman holding a security guard at his side with other hostages lying on the floor of the lobby.

"We couldn't see the weapon. We peered in and could make out his silhouette. We saw him holding a security guard at his side. There were other people on the floor, a dozen or so," Fisher told ABCNews.com from locked building across the street.

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Discovery Gunman James Lee's 2008 Arrest

Others were trapped inside the suburban headquarters.

"Friends, I am in work closet with workers here. There is a shooter here at Discovery. I am ok," read a text from one Discovery employee.

Another employee said the Lee had taken a hostage. "The police are trying to get the situation under control!INSANE!" read the email to ABCNews.

Some employees were initially sent to higher floors and later evacuated, police said. Witnesses on the scene said another group of employees left the building around 1:44 p.m.

The building has a daycare center, which witnesses say has been evacuated.

"Crazy to see the babies coming out of childcare center piled into rolling cribs into the McD's across the street, a safe zone," said a witness via email.

The building, which houses the offices of Discovery Channel and its sibling networks including Animal Planet, is located just outside Washington DC.

Lee operates the Web site savetheplanetprotest.com and was arrested in 2008 following a demonstration held at Discovery's headquarters. Lee was found guilty of disorderly conduct.

According to a post on his Web site, Lee said he was thrown "in the nut house" for four days following his arrest.

Lee has been demanding a boycott of Discovery Channel since at least 2008.

Discovery Channel Gunman Penned Manifesto

On his website, Lee posted a rambling manifesto under the title "My Demands," which espouses a far-radical environmentalist and misinthropic philosophy and calls on the channel to cease programming about giving birth, war and weapons.

"The planet," he wrote, "does not need humans."

The document, which appears to have been created on July 17, is interspersed with references to esoteric philosophers, childish language, misspellings, and capital letters.

Lee writes that the channel should cease its current programming and replace it with a game show about reducing the global population.

The channel, he wrote, should produce a program about "how people can live WITHOUT giving birth to more filthy human children since those new additions continue pollution and are pollution."

"The world needs TV shows that DEVELOP solutions to the problems that humans are causing, not stupify the people into destroying the world. Not encouraging them to breed more environmentally harmful humans," he wrote.

"Saving the environment and the remaning [sic] species diversity of the planet is now your mindset. Nothing is more important than saving them. The Lions, Tigers, Giraffes, Elephants, Froggies, Turtles, Apes, Raccoons, Beetles, Ants, Sharks, Bears, and, of course, the Squirrels. . . The humans? The planet does not need humans," he wrote.